This invention was made with government support under Cooperative Agreement: NCC2-9016 for the Variable Geometry Advanced Rotor Technology program awarded by NASA. The government therefore has certain rights in this invention.
The present invention relates to an active multi-element rotor blade, and more particularly to controlling an active slat relative to a main element.
Multi-element airfoils are in common use on fixed wing aircraft. Such applications, however, are either in a fixed configuration or activate at relatively slow rates. In conventional applications, the aerodynamic flow environment is steady or quasi-steady.
Multi-element airfoil application to rotary-wing aircraft has concentrated upon the development of fixed elements which attempt to provide a compromise between achieving an average improvement to rotor disc lift and avoiding an unacceptable increase in drag. Such fixed elements provide numerous design challenges including the aerodynamic requirements from lower-speed, high angle of attack on the retreating side of the rotor disc to high speed, low angle of attack operation on the advancing side of the rotor disc. Current designs for high lift in the low speed regime suffer from unacceptable drag levels at high speed while current designs for low drag in the high-speed regime do not show sufficient benefits of increased lift in the low speed regime.
Accordingly, it is desirable to provide an active multi-element rotor blade airfoil which is configurable to maximize lift performance while minimizing drag in various flight regimes.
The present invention provides a multi-element rotor blade having a main element and an active slat movable relative to the main element. The slat rotates and translates relative to the main element from a base position. The base position provides a compromise between minimum coefficient of drag CD and maximum coefficient of lift CLmax. During forward flight a rotor blade will either be advancing or retreating such that the forward velocity is added to or subtracted from the rotor angular speed. For the advancing blade, the airspeed thereof is significantly greater than the retreating blade. Applicant has determined that a positive rotation of the slat pitch angle from the base position minimizes drag at low angles of attack, providing a coefficient of drag CD lower than a conventional single element rotor blade. For the retreating blade, the airspeed thereof is significantly lower than the advancing blade, Applicant has determined that negative rotation and translation of the slat maximizes the coefficient of lift CLmax.
In one control method, the slat is passively controlled in accordance with azimuth angle xcexa8. The slat is driven between low drag and maximum lift positions in a sinusoidal or other prescribed wave pattern which has a minimum amplitude near xcexa8=90 degrees and a maximum near xcexa8=270 degrees. That is, the slat is at the low drag position near xcexa8=90 degrees, at the maximum lift position near xcexa8=270 degrees and at the base position near xcexa8=0, 180, and 360 degrees per rotor revolution.
According to another control method, the slat is independently controlled by a controller which provides full independent slat control with a feedback system to drive the slat to calculated positions in accordance with an aircraft flight control system. That is, the slat is selectively actuated to a particular position according to the flight control system.
The present invention therefore provides an active multi-element rotor blade airfoil which is configurable to maximize lift performance while minimizing drag for all flight regimes.